In automation technology, drives are frequently used whose motors are controlled by a numerical control (NC). To that end, depending on a program to be processed, the control generates setpoint values which are converted in suitable motor-control modules, referred to as converters, to form control signals for the motor. The movement resulting from the control signals may be both a longitudinal movement, e.g., the travel of a tool carriage in a machine tool, and a rotary movement, e.g., a tool spindle rotating with a specific speed or the rotation of an articulated joint of a production robot.
In order to be able to measure the extent of the movement, position-measuring devices are used. In the case of a longitudinal movement, linear position-measuring devices, for example, and in the case of rotary movements, rotary position-measuring devices, also referred to as rotary transducers, are used, which are coupled directly or via a speed-transforming gear to a motor shaft. At regular time intervals, referred to as the controller cycle time, the control retrieves actual position values from the position-measuring devices and uses these values to ascertain new setpoint values for the converters. Control loops of this kind permit precise control of the drive.
The position values may be transmitted from the position-measuring devices to the control in purely analog fashion, often in the form of two sinusoidal signals phase-shifted by 90° relative to each other, or also digitally in the form of square-wave counting signals, or by transmitting complex data words via data interfaces. In modern position-measuring devices, serial data interfaces are considered to be preferred, since they require only a small number of lines for the data transmission, and permit the transmission of absolute position values.
To ascertain the setpoint values for the control loops, in addition to needing the actual position values, the drive control also needs further movement data such as the instantaneous velocity, or rotational speed, or perhaps the acceleration. If the controller cycle time is known, the velocity may be derived from two successively measured position values by forming the difference quotient. In a similar manner, the acceleration may be ascertained with the aid of three successively measured position values, or two successively calculated velocity values. However, motion values calculated in this manner represent only an average value with which the controlled drive has covered the distance from the first to the second and possibly the third position.
In practice, the actual velocity of a drive at a specific measuring instant may deviate considerably from the average value. For example, this may have mechanical causes, such as imbalances in the motor shafts, frictional effects or load change produced, for example, by the engagement of a tool with a workpiece. However, particularly problematic are errors of the position-measuring device, especially quantization errors, since the shorter the controller cycle time and the smaller the difference between two successively measured position values resulting therefrom, the stronger the effect of these errors. If the average value, which is based on successively measured position values, is used for ascertaining a new velocity setpoint value, because of the discrepancy between the actual velocity at the measuring instant and the average value, undesirable control fluctuations can occur which can lead to heating of the drive motor, to annoying noise generation, or perhaps to vibrations due to resonance effects.
Modern position-measuring devices, especially those which, by the use of large-scale integrated signal-processing units, e.g., in the form of a microcontroller or corresponding structures in an application-specific integrated component (ASIC), are able to carry out complex calculating operations, often themselves can already ascertain movement values, particularly velocity and acceleration values, with high accuracy. However, since in this case both the instantaneous position value and at least one movement value, for example, the velocity value, must be transmitted from the position-measuring device to the control, assuming a constant controller cycle time, the time available to the control after receiving the instantaneous actual values to perform the necessary calculations for ascertaining the new setpoint values is reduced. For this reason, especially when the data is transmitted via serial interfaces, this variant is often not usable because an increase in the data-transmission rate is associated with a high expenditure in material and costs.